1. Field of the Invention
The present invention relates to an assembly structure for an optical integrated circuit device comprising a semiconductor laser device secured to an optical integrated circuit substrate, and a method for assembling the same.
2. Description of the Related Art
An optical integrated circuit device is one of the basic devices in the fields of optical communications, optical information processing, optical measurement, etc. Among others, an optical integrated circuit device using a semiconductor laser as a light source is indispensable for the realization of ultra-high-speed optical communications, super parallel optical information processing, high-precision optical measurement, etc.
FIG. 1 is a simplified schematic diagram showing an example of an optical integrated circuit device 90 having a semiconductor laser device 80 as a light source. In this optical integrated circuit device 90, the semiconductor laser device 80 and a waveguide device 5 having a waveguiding structure for controlling light waves are formed on a common substrate 50. In building such an optical integrated circuit device 90, design and fabrication of the semiconductor laser device 80 and the waveguide device 5 can be made easier by forming these two devices 80 and 5 using different materials rather than integrating them using an identical material.
Accordingly, it is common practice to form the semiconductor laser device 80 and the waveguide device 5 separately, and then mount them in place on the common substrate 50. The most important thing in this process is to ensure enough optical coupling efficiency in coupling between a light-emitting layer 11 in the semiconductor laser device 80 and an optical waveguiding layer (simply referred to as the waveguide hereinafter) 6 in the waveguide device 5.
In the above mounting process, the semiconductor laser device 80 and the waveguide device 5 need to be aligned with each other. The alignment is usually done in one of the following two ways. One involves an optical method in which light emitted from the semiconductor laser device 80 is converged by a lens to the waveguide 6 in the waveguide device 5. The alignment is adjusted by checking whether a prescribed light collection rate is achieved. The other is a butt-coupling method in which the light-emitting layer 11 in the semiconductor laser device 80 and the waveguide 6 in the waveguide device 5 are arranged while butting each other so that the two layers 6 and 11 are directly coupled with each other.
The optical method using a lens requires alignment among three devices, i.e., alignment among the semiconductor laser device, the waveguide device, and the lens. Accurately aligning these devices to one another to complete a chip is an extremely difficult job. Therefore, the butt-coupling method is often used in preference to the optical method.
FIG. 2 is a schematic diagram for explaining a method of assembling the optical integrated circuit device 90 by coupling the semiconductor laser device 80 and the waveguide device 5 together by the butt-coupling method.
In this method, light emitted from the semiconductor laser device 80 is directly fed into the waveguide 6 of the waveguide device 5. The light 12 exiting the waveguide 6 is received by a light detector 10 which detects the intensity of the light 12. The optimum relative positioning between the semiconductor laser device 80 and the waveguide device 5 is determined as the point where the largest current value is detected by the light detector 10.
In this butt-coupling method, the semiconductor laser device 80 is driven to emit light for coupling to the waveguide device 5, as described above. To prevent degradation of the semiconductor laser device 80 by the heat generated during the light emission, the semiconductor laser device 80 is held in intimate contact with a heat sink substrate 1. On the other hand, the waveguide device 5 is placed in a movable state.
The above butt-coupling method will be described in further detail below.
First, the semiconductor laser device 80 is held firmly in intimate contact with the surface of the heat sink substrate 1, which is made of silicon or the like, and the heat sink substrate 1 is secured to a stem 2. The semiconductor laser device 80 is connected by an electrode wire 3 to an electrode pin 4 provided on the stem 2. A current is fed from the electrode pin 4 to the semiconductor laser device 80 through the electrode wire 3, and this current drives the semiconductor laser device 80 to emit light.
Next, with the semiconductor laser device 80 continuously driven to emit light, the waveguide device 5 in chip form is placed on the stem 2 and moved toward the semiconductor laser device 80. The waveguide device 5 has a multilayered structure with the waveguide 6 formed at the same height as the light-emitting layer 11 of the semiconductor laser device 80. While the waveguide device 5 is being moved, the light emitted from a light-emitting part 8 of the semiconductor laser device 80 enters the waveguide 6 through a light input facet 7. The incident light propagates through the waveguide 6 and emerges from a light exit facet 9 of the waveguide 6. This emergent light 12 is received by the light detector 10 to detect the intensity of the light 12. The waveguide device 5 is incrementally moved in three crossed directions, and the position where the largest current value is detected by the light detector 10 is determined as the optimum mounting position.
Once the optimum alignment is achieved, an ultraviolet curing resin (not shown) is applied over the waveguide device 5. This resin is then cured by exposure to ultraviolet radiation, to fix the waveguide device 5 in position.
However, the above-described prior art has the following problems.
Firstly, since the waveguide device 5 has to be moved along the heat sink substrate 1 for alignment, the waveguide devices 5 as fabricated on a wafer cannot be handled. A wafer containing many waveguide devices 5 has to be separated into chips first, and alignment work has to be performed on each individual device separated as a chip. The result is low productivity.
Secondly, the process involved is very complex.
Thirdly, for obtaining maximum optical coupling efficiency, not only the positioning accuracy in the direction parallel to the layers of the semiconductor laser device 80, but the tilting within a plane perpendicular to the layer containing the optical axis of the light-emitting layer 11 of the semiconductor laser device 80 needs to be controlled. Generally, the positioning accuracy required in butt-coupling two waveguides is 1 .mu.m or less in either of two directions perpendicular to the waveguide optical axis (one is a thickness direction across the thickness of the layer forming the waveguide, and the other is a direction perpendicular both to the thickness direction of the layer forming the waveguide and to the waveguide optical axis). In particular, the alignment in the thickness direction of the layer forming the waveguide demands an extremely high accuracy of 0.5 .mu.m or less. In the above prior art method, since the waveguide in chip form has to be moved while constantly monitoring the current value being detected by the light detector, not only the productivity but the fabrication yield suffers.